This invention relates to vehicle detector systems used to detect the presence or absence of a motor vehicle over an inductive loop embedded in the pavement. More particularly, this invention relates to a vehicle detector system with an improved loop oscillator circuit
Vehicle detectors have been used for a substantial period of time to generate information specifying the presence or absence of a vehicle at a particular location sometimes termed a detection zone. Such detectors have been used at intersections, for example, to supply information used by an associated traffic control unit to control the operation of the traffic signal heads, and have also been used to supply control information used in conjunction with automatic entrance and exit gates in parking lots, garages and buildings. A widely used type of vehicle detector employs the principle of period shift measurement in order to determine the presence or absence of a vehicle in or adjacent to the inductive loop mounted on or in a roadway. In such systems, a first oscillator-termed the loop oscillator-, which typically operates in the range from about 20 kHZ to about 100 kHZ, is used to produce a periodic signal in a vehicle detector loop. A second oscillator operating at a much higher frequency is commonly used to generate a sample count signal over a fixed number of loop oscillator cycles. The relatively high frequency count signal is typically used to increment a counter, which stores a number corresponding to the sample count at the end of the fixed number of loop oscillator cycles. This sample count is compared with a reference count stored in another counter and representative of a previous count in order to determine whether a vehicle has entered or departed the region of the loop in the time period between the previous sample count and the present sample count.
The initial reference value is obtained from one or more initial sample counts and stored in a reference counter. Thereafter, successive sample counts are obtained on a periodic basis, and compared with the reference count. If the two values are essentially equal, the condition of the loop remains unchanged, i.e., a vehicle has not entered or departed the loop. However, if the two numbers differ by at least a threshold amount in a first direction (termed the Call direction), the condition of the loop has changed and may signify that a vehicle has entered the loop. More specifically, in a system in which the sample count has decreased and the sample count has a numerical value less than the reference count by at least a threshold magnitude, this change signifies that the period of the loop oscillator signal has decreased (since fewer counts were accumulated during the fixed number of loop oscillator cycles), which in turn indicates that the frequency of the loop oscillator signal has increased, usually due to the presence of a vehicle in or near the loop. When these conditions exist, the vehicle detector generates a signal termed a Call signal indicating the presence of a vehicle in the loop.
Correspondingly, if the two numbers differ by less than a second threshold amount in a second direction (termed the No Call direction), this condition indicates that a vehicle which was formerly located in or near the loop has departed the detection zone. When this condition occurs, a previously generated Call signal is dropped.
The difference ΔN between a sample count N and a reference count R is representative of the inductance change in a loop oscillator circuit at the end of the time period between the former sample count (the reference count R) and the current sample count N. More particularly, the quantity ΔL/L=k ΔN/N, where L=loop inductance and k is a scaling factor, expresses the relationship between numerical counts and loop inductance.
Call signals are used in a wide variety of applications, including vehicle counting along a roadway or through a parking entrance or exit, vehicle speed between preselected points along a roadway, vehicle presence at an intersection controlled by a traffic control light system, or in a parking stall, and numerous other applications.
In addition to the basic function of generating and dropping a Call signal, existing vehicle detectors incorporate other features, some of which are selectable on-site by a technician. For example, some vehicle detectors incorporate an end of green function which requires the detector to automatically reset after the green traffic signal, which controls the lane in which the loop associated with the vehicle detector is located, terminates. Some vehicle detectors are provided with an extension time feature which extends the Call signal for a period of time after a vehicle leaves the associated loop (typically in order to permit ample minimum time for a vehicle to clear an intersection). Some vehicle detectors are also provided with a presence/pulse selection feature, which causes the vehicle detector to generate one of two types of Call signals: a continually persisting signal so long as the vehicle remains in the loop (the presence function); or a fixed length pulse generated when the vehicle is first detected in the loop, or when the vehicle departs the loop (the pulse function). Still other vehicle detectors are provided with selectable different sensitivity settings, which enable a technician to adjust the response of the vehicle detector when connected to the loop in order to accommodate a range of detection conditions.
In the past, vehicle detectors have been designed as either single channel or multiple channel detectors. A single channel detector is designed and configured to operate with only a single loop zone; while a multiple channel vehicle detector is designed and configured to operate with two or more independent loop zones. Multiple channel detectors are designed to be either scanning or non-scanning detectors. A scanning detector operates by sampling only one loop channel at a time, shutting down the active loop, sampling the next loop channel, shutting down that loop, etc. Scanning detectors are typically used in installations in which the probability of cross-talk between loop circuits is more than minimal. Cross talk results when physically adjacent loops are operating at, or near, the same frequency. Cross talk is minimized or eliminated by operating physically adjacent loops on different frequencies. Non-scanning vehicle detectors are configured and function to monitor each of the multiple loop zones simultaneously. Non-scanning detectors are typically used in installations in which there is a very low or no possibility of cross-talk between the multiple loop circuits, such as installations at which the loops are physically separated by a distance sufficient to ensure no overlapping or inter-coupling between the electrical fields associated with the loops. An example of a vehicle detector incorporating the functions described above is disclosed in U.S. Pat. No. 6,087,964 issued Jul. 11, 2000 for “Vehicle Detector With Operational Display”, the disclosure of which is hereby incorporated by reference.
When deployed in an intersection controlled by a traffic control light system, vehicle detectors generate signals which are used by the intersection traffic controller to supervise the operational states of the traffic control heads in response to the arrival and departure of vehicles over loops installed in the various lanes leading to the intersection.
A key component of vehicle detectors is the loop oscillator circuit. This circuit includes oscillator components capable of generating the periodic signals noted above, a transformer to which the oscillator components are coupled, and the loop which typically comprises a closed loop of one or more turns embedded in the roadway surface and a pair of electrical conductors connected between the ends of the loop and one side of the transformer. While the oscillator components and the transformer can be mounted to a circuit board housed within a control unit cabinet and thereby somewhat shielded from the outside environment, the loop and the interconnecting electrical conductors of necessity are directly exposed to the outside environment and consequently are directly subject to changes in environmental conditions, such as wide variations in temperature and humidity, as well as mechanical vibrations due to vehicle traffic, construction work, seismic earth movements and the like. These changes have a direct impact on the functional stability of the loop oscillator circuit, which operates in the analog domain. More particularly, changes in environmental conditions can adversely affect the amplitude of the loop oscillator circuit to such an extent that one or more loop cycles can be missed by the loop counter, or one or more phantom loop cycles can be erroneously counted by the loop counter. In either case, the resulting accumulated sample count will not accurately reflect whether or not the status of the loop circuit has actually changed, and the vehicle detector may erroneously generate a false Call signal or erroneously drop an existing Call signal. Known vehicle detector oscillator circuits are not designed to compensate for this severe disadvantage.
Known vehicle detector oscillator circuits suffer from an additional disadvantage. In many intersections where vehicle detector oscillator circuits are deployed there are oscillator loops located in adjacent vehicle lanes, each connected to individual oscillator circuits which function independently of each other. When the operation of a given oscillator circuit is terminated at the end of a sampling period, that oscillator circuit does not cease generating a loop oscillator signal immediately, but continues to resonate for a few cycles due to a phenomenon known as “ringing”. This “ringing” phenomenon can carry over to the oscillator circuits in adjacent vehicle lanes by virtue of cross-talk between the ringing oscillator circuit and the adjacent lane oscillator circuits, which interferes with their operation and can cause erroneous results in those signals.